The rates of uptake, oxidation, and incorporation into lipids of radioactive fatty acids in "high" and "low" lipid cells will be investigated over longer periods of time than hitherto studied in order to elucidate the anomalous observation that low lipid cells take up and incorporate fatty acids into lipids of a faster rate than low lipid cells. In addition, the excretion of the labeled complex lipids into the medium will also be examined in long term experiments and the identity of such lipids, if they exist, will be determined. Experiments will be carried out, using the isotopic labeling technique, to identify the lipoprotein lipids, or their partial hydrolysis products, that appear to contribute substantially to the accumulation of triglycerides and even more extensively to the formation of phospholipids in dividing cells.